I had similar issues with mine. Took it to the LBS for a bleed and it was as you explained, better but still not as fast as it should be. I rode it for a week then talked to the mechanic. He had another go and its as good as new now.
I have had similar challenges with Avid brakes. It always takes me a few goes to get them sweet.
Crank Cycles in Alton Hants deserve a plug here. Sam is a top guy!
Had very similar problems. Was working fine, then would not return unless I had to pull it. Sometimes it would move a little but then stop.
I bled it countless times and wasn’t getting anywhere. This time I bled slightly differently, instead of pushing fluid from both ends, I bled from one end only. I wound the speed adjuster in, then connected the bleed kits. The syringe on the bars was filled 3/4’s full and just about 1/5 full on the seatpost side. I then bled from the bars to the seatpost only. Once the syringe on the bars was just about empty, I swapped the two syringes over (keeping the hose part of the bleed kit still attached to the bleed ports). When reconnecting the syringe for on the remote side, I would then press the button. Pushing any air from my connection back into the syringe. I must have pushed through about 4 syringes full.
I don’t know if the above would work with stale oil, I’ve bled it so many times recently that I knew all the oil inside was new. Though the above tips on having the hose straight makes lots of sense.
I also found Juicy Lubes fork juice helps keep it running well
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